1. Field of the Invention
The present invention relates to a means and method for removing airborne particulates from an aerosol, and more particularly to unique and simplified means and method for enhancing the growth of said particulates whereby to efficiently and economically remove same utilizing charged droplet scrubbing techniques.
2. Brief Description of the Prior Art
It is known in the art to use an electric field to sweep highly charged droplets, i.e.--charged within a factor of their maximum theoretical limit, across an aerosol whereby to cause the charged droplets to collect ariborne particulates and remove same from the aerosol, so-called electrostatic scrubbing. As taught by Richards, U.S. Pat. No. 4,095,962, particulate collection efficiency may be enhanced if the airborne particulate has been charged, prior to electrostatic scrubbing, with a polarity opposite that of the highly charged droplets. Most small particulates, in the order of 5 microns or less, which typically escape from conventional electrostatic precipitators will thus carry a small net opposite charge whereby to obtain coulomb-enhanced coalescence of the particulates and the droplets.
However, practical difficulties in charging small particles, particularly those particles having a high resistivity, taken together with the increased complexity of the apparatus if an initial charge is to be imparted to the particulates, detracts somewhat from the adaptability of Richards for scrubbing airborne particulates emitted by certain industrial processes.
Present attempts to evolve improved scrubbers having enhanced collection efficiencies for removing small particulates have been primarily directed towards the development of flux force condensation (FF/C) scrubbers. Briefly, these scrubbers employ the effects of thermophoresis, diffusiophoresis, and condensation to effect particle growth and move said particles towards a wetted surface. The prior art recognizes that particle enlargement due to water vapor condensation thereon enhances inertial impaction of the particles. However, the prior art has hitherto assumed that if a critical saturation ratio, which is a function of particle size and wettability and solubility in water, is achieved, spontaneous nucleation, condensation, and particle growth can be obtained. This assumption is based upon "cloud formation" theory wherein the particle loading number density per unit of air is quantitatively much smaller than the particle loading number density typically encountered in aerosols discharged from various industrial processes.
With regard to condensation as a means for effecting particulate growth, it is known in the art to saturate a gas stream, having a temperature above 212.degree. F., with a fine mist spray of water. The saturated gas stream is adiabatically cooled whereby water is precipitated upon airborne particles carried in the gas stream, and then conventional cyclone separators are utilized to remove said particles. C. H. Hausberg, et al., U.S. Pat. Nos. 3,773,472 and 3,906,078. The drawbacks associated with such a method are substantial. By using water as the sole means for saturating the gas stream, and then allowing for a residence time sufficient to establish an equilibrium temperature, some water will evaporate into the gas stream thereby cooling it. As a result the amount of water vapor that can be carried by the stream is limited and significant pressure drops within a treatment system are required downstream of the saturation step in order to condense from said gas stream the quantity of water necessary to grow the particulate to a size sufficient for removal. A still further drawback resides in the fact that saturation and subsequent cooling of the aerosol alone is inadequate to stimulate the growth of submicron particulates and, therefore, these particulates generally avoid being collected.
In view of the hereinbefore described limitations present in the prior art, it is an object of the present invention to provide a method for economically and efficiently removing airborne particulates from an aerosol.
A still further object of the present invention is to provide a method for enhancing submicron particulate growth to a size sufficient for facilitating the removal thereof from an aerosol.
Yet another object is to provide a method for scrubbing aerosols adaptable over a wide range of particulate loading densities within the aerosol.
Another object is to provide a method particularly adapted for removing particulates from aerosols with minimum system energy losses.
A still further object is to provide a method for enhancing particulate growth to a size sufficient for removal by electrostatic scrubbing techniques.
And yet another object is to provide a new and unique apparatus useful in scrubbing particulates from an aerosol.
These and other objects of the present invention will be readily appreciated by reference to the following description of the present invention taken in conjunction with the accompanying drawings.